The present invention relates to an image forming apparatus of the type uniformly charging a photoconductive element, exposing the charged surface of the photoconductive element imagewise to electrostatically form a latent image thereon, and developing the latent image by a toner or similar developer. More particularly, the present invention is concerned with an image forming apparatus which senses the surface potential of the photoconductive element by a sensor and controls an image forming condition or conditions on the basis of the sensed surface potential.
A copier, laser printer and facsimile transceiver belong to a family of image forming apparatuses of the type described. With a copier, for example, it is a common practice to illuminate the charged surface of a photoconductive element by light representative of a document image. To enhance the quality of a reproduced image, the copier senses the surface potential of the photoconductive element by a non-contact type sensor and controls the bias for development or similar image forming parameter in matching relation to the sensed surface potential. The problem with such a copier is that the distance between the sensor and the surface of the photoconductive element and, therefore, the output of the sensor is not constant due to mechanical irregularities. Should the image forming parameter be controlled on the basis of such an output of the sensor, the resultant image would suffer from the deviation in notch.
In light of the above, a non-contact type sensor having a distance compensating circuit has been proposed in the past. However, even the distance compensating circuit is not fully free from the influence of the irregularity in the distance between the sensor and the photoconductive element. In addition, an image forming apparatus with such an extra circuit would be expensive.
To promote the accurate measurement of a surface potential, a particular reference potential area may be provided on the surface of the photoconductive element, as disclosed in Japanese Patent Laid-Open Publication No. 55356/1980. The surface potential of the reference potential area is sensed by a non-contact type potential sensor, while the potential of the other area is sensed by another non-contact type potential sensor. Then, the surface potential of the photoconductive element is determined on the basis of the result of differential amplification of the sensed two surface potentials. This approach, however, brings about another problem that part of the surface of the photoconductive element is occupied by the reference potential area and cannot join in the image forming operation at all, i.e., the surface of the element cannot be effectively used.